Method of coating a metallic mold surface with a boron containing compound



United States Patent 3 342 249 METHOD OF COAT INd A METALLIC MOLD SURFACE WITH A BORON CONTAINING This application is a continuation application of our copending application, Ser. No. 206,178, filed June 29, 1962, and now abandoned.

Thisinvention pertains to the art of coating the inner surface of a mold for molten metal and more particularly to an improved compound for coating the inner surfaces of a molten metal mold and the method of applying the compound onto these surfaces.

The invention is particularly applicable to coating the metal inner surfaces of an ingot mold used for molding an ingot of ferrous material, and the invention will be discussed with particular reference thereto; however, it is to be appreciated that the invention has much broader applications and may be used for the coating of the metal inner surfaces of the various molten metal molds such as those used in casting zinc, aluminum and other nonferrous metals. I

In the art of casting ferrous ingots, for instance ingots of steel preparatory to rolling or other hot treatment, it is important that the outer surface of the ingot have a substantially smooth surface so that it is not necessary to scarf the outer surface before the ingot is fed intothe rolling mill. To obtain a smooth outer surface on the steel ingot, it has become common practice to coat the metal inner surface of the ingot mold with various material. For instance, the metal inner surfaces of the ingot mold have been coated'by suspensions of various finely divided inorganic substances, such as graphite, silica flour, and lime to name only a few. These substances did not form a good bond with the inner surface of the mold and the steel poured into the mold tended to wash away the coating which completely destroyed the usefulness of the coating. To correct these difiiculties, it has been proposed to form in situ by pyrolyzing, whollyor in part, heavy organic material, like tar or pitch, on the inner surface of the mold. However, the very nature of these materials, such as their high viscosity, prevented these materials from forming an effective, continuous, unbroken film on the inner surface ofthe mold'to separate the mold from the molten metal therein. As in the case of the inorganic suspension, the heavy organic materials were easily washed from the inner surface of the mold so that these organic materials formed an imperfect barrier between the mold and the molten metal. Without a substantially continuous separating barrier between the mold and the molten metal, the molten metal in direct contact with the mold tended to form cold shuts along the surface of the ingot. Also, if the coating of the ingot mold was not continuous, the molten metal tended to adhere to the ingot mold sothat surface defects would result as the mold was separated from the ingot. These cold shuts and other surface defects necessitated hand chipping, scarfing and grinding before the ingot was in proper condition for the rolling mill. Also, the manual preparation of the outer surface often required additional cooling and heating of the ingot before the ingot could be rolled into sheets which created storage problems and increased the processing cost of the steel. It is appreciated, that these same defects occurred in casting non-ferrous metals such as aluminum and zinc when the surfaces of the molds were not properly treated.

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These and other disadvantages of prior coating compounds are eliminated by the present invention which is directed toward a mold coating compound and method of applying the same which forms a thin continuous film on the inner surfaces of a mold used for casting a molten metal so that the cast metal has a smooth outer surface and is easily removable from the mold without causing major surface defects.

In accordance with the present invention there is provided a coating compound for forming a continuous film on the inner metal surfaces of a mold for molten metal which compound comprises a boric oxide. The compound is of the type formed by oxidizing methyl borate, ethyl borate, propyl borate, isopropyl borate or mixtures of these borates. Such oxides are boric oxide (B 0 boron suboxide (B 0 orthoboric acid (H BO metaboric acid '(HBO and hydrates of the boric acids.

In accordance with another aspect of the present invention there is provided a method of coating the inner surfaces of a molten metal mold with a continuous film comprising the following steps: heating the inner surface of the mold, and applying an oxidizable boron containing compound onto the inner surface.

The primary object of the present invention is the provision of a coating compound to coat the inner surfaces of a mold used to cast a molten metal which compound,

forms a continuous film on the mold surfaces so that the outer surface of the metal cast in the mold is substantially smooth.

Another object of the present invention is the provision of a coating compound to coat the inner surfaces of a mold used to cast a molten metal which compound forms a continuous film on the surface that can withstand splashing of the metal poured into the mold.

Still another object of the present invention is the provision of a coating compound to coat the inner surfaces of a mold used to cast a molten metal which compound comprises an oxide of boron.

Yet another object of the present invention is the provision of a coating compound to coat the inner surfaces of a mold to cast a molten metal which compound is formed by oxidation of a boron compound.

Another object of the present invention is the provision of a coating compound to coat the inner surfaces of a mold used to cast "a molten metal which compound is formed by oxidation of methyl borate, ethyl borate, propyl 'borate or isopropyl borate or a mixture of these borates.

Still another object of the present invention is the provision of a method of coating the inner surfaces of a molten metal mold with a thin continuous protective film which provides a smooth surface on the metal cast in the mold.

Another object of the present invention is the provision of a method of coating the inner surfaces of a molten metal mold with a thin continuous protective film which film will withstand splashing of the metal against the surface.

Yet another object of the present invention is the provision of a method of coating the inner surfaces of a molten metal mold with a thin continuous protected film of a boric oxide.

Still another object of the present. invention is the provision of a method of coating the inner surfaces of a molten metal mold with a thin continuous protected film which film is formed by oxidizing a boron compound.

Another object of the present invention is the provision of a method of coating the inner surfaces of a molten metal mold with a thin continuous protected film which film is formed by the oxidation of methyl borate, ethyl borate, propyl borate or isopropyl borate or a mixture of these borates.

In accordance with the invention, a thin continuous film or layer of an oxy-boron compound is deposited onto the inner surfaces of the casting mold, prior to casting the molten metal therein. The composition of this film or layer may vary according to the technique of application, from a completely hydrated composition, such as H BO to a completely anhydrous composition such as B or from a hydrated suboxide of boron to completely anhydrous suboxide of boron. Under most conditions wherein the oxide is deposited on the inner surfaces of the mold, the normal oxide produced is the substantially anhydrous oxide; however, it is to be appreciated that both anhydrous and hydrated oxides are within the contemplation of this invention. The primary feature of the boric oxide coating is that it forms a substantially continuous coating or film on the inner surfaces of the mold which coating or film cannot be readily separated from the surfaces. Accordingly, the mold surfaces are separated from the molten metal poured into the mold during the casting operation.

The term oxy boron compound, as used herein, is any compound resulting from the union of oxygen and boron, or the compound resulting from the union of oxygen, hydrogen and boron. Accordingly, this term oxy-boron compound, includes but is not limited to boric oxide (B 0 boron suboxide (B 0 orthoboric acid (H BO metaboric acid (HBO and other hydrates of boric acids. These oxides are formed by oxidizing such boron compositions as methyl borate, ethyl borate, propyl borate, isopropyl borate or a mixture of these borates. It is to be appreciated that a wide variety of other oxides and su boxides of boron either hydrated or anhydrous can be used to form the continuous film or coating which is deposited on the inner surfaces of the mold.

The method of forming the continuous film or coating on the inner surfaces of a molten metal mold comprises basically the step of subjecting the inner surfaces of the mold to an oxidizable boron compound and oxidizing this compound to deposit an oxy-boron compound onto the inner surfaces of the mold. Although a variety of methods are within the contemplation of the invention, in accordance with the preferred embodiment, the method of forming the oxy-boron coating or film on the inner surfaces of the mold comprises impinging a gaseous flame onto the surface, and providing the flame with a vaporized or suspended organic boron compound capable of being oxidized to form the required oxyboron composition. For example, a natural gas torch may be used with methyl borate, ethyl borate, propyl 50 borate, isopropyl borate or a mixture of these borates entrained in the gas stream. It is appreciated that pure methyl borate or its methanol azetrope may be used equally well in the carrier flame. Combustion of the vapor carrying gas oxidizes the organic borate at a temperature sufficiently high to deposit a continuous film or coating of oxy-boron composition on the inner surface of the mold on which the flame is impinging. This coating is deposited in a nascent condition and therefore is usually active, so that it forms a strong adhesion to the inner surface of the mold. It has been found that the resulting film establishes an excellent parting barrier for the mold even though the film is quite thin. For instance, it has been found that steel poured into an ingot mold, treated as thus explained, parts readily from the ingot mold with an excellent outer surface so that the ingots may be immediately rolled into sheet form. It is believed that organic borates carried in an impinging flame have never before been used to create a parting film on the inner surfaces of a molten metal mold.

In accordance with a slight modification of the preferred embodiment, the oxy boron film or coating is deposited onto the inner surfaces of a molten metal mold by spraying an oxidizable boron compound directly onto the inner surface of the mold after the surface has been heated, and thereafter or simultaneously subjecting the mold to an oxidizing atmosphere such as oxygen or air to allow combustion of the boron compound. The

5 temperature of the mold must be sufficient to ignite the organic boron compound and thereby deposit onto the inner surfaces of the mold the desired oxy-boron film or coating. A similar result may be achieved by brushing or pouring the organic boron compound such as methyl borate onto the inner surface of the mold after the surface has been heated so that the temperature of the surface ignites the borate and deposits an oxy-boron film onto the surfaces.

Even a further method of depositing the oxy-boron compound onto the inner surfaces of a mold comprises spraying or brushing a solution of the oxy-boron compound onto the surfaces of the mold. This oxy-boron compound may be carried in a solvent such as alcohol whereby the solvent is combustible to assist in depositing a continuous oxy-boron coating onto the inner surfaces of the mold. If water or other non-combustible solvents were used for the oxy-boron compound, the mold or other surface may be maintained at a temperature well over the boiling point of water or other non-combustible solvent 5 so that the temperature of the mold acts to volatilize the solvent to effect pyrolysis, wholly or partially of the compound to yield the desired oxy-boron continuous film on the inner surfaces of the mold.

Still a further modification of the method for depositing the oxy-boron coating onto the surface of a mold comprises pouring the molten metal into the mold through a curtain of burning gas or fuel containing vaporized or suspended oxy-boron material. The molten metal, falling through the curtain or cloud of vaporized oxy-boron compound in the flame, mechanically carried enough boron compounds to the inner surfaces of the mold.

Still further, the mold may be heated and dipped into a solution of boric acid or other boron compound to obtain the oxy-boron film on the inner surfaces of the mold.

Various other methods could be devised for depositing the oxy-boron compound onto the inner surface of the molten metal mold and the invention is not to be limited specifically to the preferred embodiment of the method as described.

By practicing the methods described, there is created an improved coating for the inner surface of a molten metal mold which functions not only as a separating film but also resists the erosion effects of the molten metal splashing against the inner surface of the mold. In one experiment, three walls of a small mold were coated with an oxy-boron surface film by means of the preferred embodiment wherein a flame carrying an organic borate was directed onto the inner surface of the mold.

The fourth wall of the mold was left untreated to act as a control wall. A relatively minor amount of metal, cast into the mold in such a way to create splashing against all four retaining walls, showed splashes adhering only to the control wall and the three oxy-boron surfaces remaining clean of adhering metal. The coating was relatively thin and the resulting cast metal had a smooth outer surface where it contacted the three coated sides.

Having thus described our invention, we claim:

1. A method of casting metal in an ingot type mold having an inner metal surface, said method comprising the following steps:

(a) directing a flame onto said inner metal surface;

(b) entraining a thermally decomposable boron containing compound in said flame;

(c) allowing said compound to oxidize on said metal surface to form a continuous layer of a boric oxide selected from the class consisting of B 0 B 0 H3BO3, and

(d) thereafter, pouring molten metal into said mold.

2. A method of casting metal in an ingot type mold having an inner metal surface, said method comprising the following steps:

(a) suspending a boric oxide in a combustible solvent;

(b) heating said inner metal surface to a temperature above the ignition temperature of said solvent;

(0) applying said boric oxide suspension onto said heated surface;

(d) allowing said solvent to burn and leave a continuous, protective film of boric oxide;

(e) thereafter, pouring molten metal into said mold.

3. A method of casting metal in an ingot type mold having an inner metal surface, said method comprising the following steps:

(a) suspending a boric oxide in a non-combustible solvent;

(b) heating said inner metal surface to a temperature above the boiling point of said solvent;

(c) applying said boric oxide suspension onto said heated metal surface;

(d) allowing said solvent to evaporate and leave a continuous, protective film of boric oxide;

(e) thereafter, pouring molten metal into said mold.

4. A method of casting metal in an ingot type mold having an inner metal surface, said method comprising the following steps:

(a) heating said inner surface;

(b) applying an oxidizable boron containing compound onto said heated metal surface;

(0) allowing said heated surface to oxidize said compound and leave a continuous, protective film; and

(d) pouring molten metal into said mold.

5. A method of coating the inner surface of a mold for molten metal with a continuous film comprising the following steps:

(a) providing a flame containing a suspension of boric oxide; and ('b) pouring molten metal through said flame into said mold.

References Cited UNITED STATES PATENTS J. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner. 

5. A METHOD OF COATING THE INNER SURFACE OF A MOLD FOR MOLTEN METAL WITH A CONTINUOUS FILM COMPRISING THE FOLLOWING STEPS: (A) PROVIDING A FLAME CONTAINING A SUSPENSION OF BORIC OXIDE; AND (B) POURING MOLTEN METAL THROUGH SAID FLAME INTO SAID MOLD. 